The present invention relates to a multibeam scanning exposure device that forms a plurality of scanning beam spots to form a drawing pattern on a photosensitive medium. Particularly, the present invention relates to the device that includes a multiple beam splitting element to divide a single light beam from a light source into a plurality of light beams.
U.S. Pat. No. 5,798,784 discloses a multibeam scanning exposure device, for example. The disclosed device is provided with an Argon laser as a light source, a half mirror to divide a light beam from the Argon laser into two beams, a pair of a multiple beam splitting elements for each of the divided light beams to further split the divided light beam into eight beams respectively and a scanning optical system that forms sixteen scanning beams on the photosensitive medium.
FIG. 8 is a perspective view showing the multiple beam splitting element that is used in an optical system of the device disclosed in the publication. The multiple beam splitting element consists of first and second right angle prisms 71 and 72 at both ends and first through seventh parallel plate prisms 73a through 73g arranged between the right angle prisms 71 and 72. These prisms are cemented with each other using organic adhesive such as epoxy resin adhesive.
Beam splitting films 74a through 74g are formed at the boundary between the first right angle prism 71 and the first parallel plate prism 73a and at the boundaries among the parallel plate prisms 73a through 73g. A reflection film 75 is formed at the boundary between the seventh parallel plate prism 73g and the second right angle prism 72.
An incident beam into the first parallel plate prism 73a from the rear side in FIG. 8 is divided into eight beams by the beam splitting films 74a through 74g, and then the parallel eight beams are exited from the first right angle prism 71 and the parallel plate prisms 73a through 73g.
Since the conventional multiple beam splitting element consists of the plurality of prisms cemented to each other, it is difficult to align the prisms in high-precision because it may include both of processing error of each prism and assembling error among the prisms. The error of the element has a bad influence on the parallelism of the exit beams, resulting in disorder of the arrangement of the scanning spots on the photosensitive medium, which deteriorates printing quality.
Further, the irradiation energy of the laser beam carbonizes the organic adhesive among the prisms of the multiple beam splitting element, lowering transmittances of the boundaries among the prisms, which loses the power balance among the eight exit beams. The conventional multiple beam splitting element has, therefore, short life span, which requires frequent exchanges of the multiple beam splitting element. Moreover, the intensity of the incident beam into the multiple beam splitting element should be controlled in low level in order to extend the life span of the multiple beam splitting element.